Large scale synthesis of 1,2,4- and 1,3,4- oxadiazole carboxylates

ABSTRACT

Disclosed are efficient and scalable processes for preparing 1,2,4- and 1,3,4-oxadiazole carboxylates from readily available starting materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an efficient and scalable process forpreparing 1,2,4- and 1,3,4-oxadiazole carboxylates from readilyavailable starting materials.

2. Description of the Related Art

Oxadiazoles are widely used as ester and amide bioisosteres. They arealso useful as antiviral agents, neuroprotectants, and anti-inflammatoryagents. An object of the present invention is to develop an efficient,scalable, and cost effective procedure for preparing multigramquantities of 1,2,4- and 1,3,4-oxadiazole carboxylates from readilyavailable starting materials.

1,2,4-Oxadiazoles can be prepared by condensing an amidoxime with anacid chloride or an acid anhydride. A limitation of this method is theavailability of the starting amidoxime, which is usually prepared bytreating a substituted oximyl chloride the highly toxic and causticammonia gas. This method also requires the use of expensive startingmaterials and cannot be used to generate a wide variety of1,2,4-oxadiazoles. In addition, this procedure is not readily amenableto large scale synthesis.

1,3,4-Oxadiazoles have been prepared by making a diacylhydrazine andtreating it with thionyl chloride and pyridine to form a1,2,3,4-oxathiadiazole-S-oxide intermediate, which then thermallyeliminates sulfur dioxide to yield the desired 1,3,4-oxadiazole. Borg,S., Estenne-Boutou, G., Luthman, K., Csoregh, I., Hesselink, W., U. B.J. Org. Chem., 1995, 60, 2112-2120. This methodology has severaldrawbacks. First, the procedure is extremely inefficient, affordingyields of 6 to 30 percent. Second, it is not amenable to scale up.Finally, this method uses acidic dehydrating agents, which will destroyany acid labile groups. Next, the preparation of the diacylhydraziderequires the use of triethylamine, a very odiferous and relativelyexpensive base. The method also requires numerous expensive and timeconsuming chromatographic separations in order to obtain the puredesired product. Finally, many 1,3,4-oxadiazoles are acid labile andreadily decompose when attempting to remove the BOC group with HCl.

Another key limitation of the prior art methods for producing the 1,2,4-and 1,3,4-oxadiazoles is that the tertiary-butoxy carbonyl (BOC)nitrogen-protecting group is left intact at the end of the synthesis.BOC groups are normally removed under acidic conditions in a proticsolvent, such as dry hydrogen chloride in saturated methanol ortrifluoroacetic acid in methanol. If these methods are used to removethe BOC group from the oxadiazole product, decomposition frequentlyoccurs, particularly in the synthesis of 1,3,4-oxadiazole systems. Theuse of toxic HCl gas is also undesirable.

A safe and efficient procedure for the synthesis of 1,2,4- and1,3,4-oxadiazoles is desired that is amenable to scale up, that affordsincreased yields, and that does not require: 1) the use of expensive,sensitive or highly toxic reagents; 2) the need for columnchromatography; 3) and the use of harsh deprotection methods.

SUMMARY OF THE INVENTION

This invention provides for efficient, scalable, safe, and costeffective methods for the preparation of 1,2,4- and 1,3,4-oxadiazoles.

Specifically, the present invention provides for a method for preparing1,2,4-oxadiazoles comprising:

-   -   A) reacting a cyanocarboxylate with a hydroxylamine salt in the        presence of a first base to form an amidoxime;    -   B) reacting the amidoxime with an acid anhydride in the presence        of a second base to afford a protected 1,2,4-oxadiazole        carboxylate; and    -   C) deprotecting the 1,2,4-oxadiazole carboxylate to form the        product.

The instant invention also provides a method for preparing1,3,4-oxadiazoles comprising:

-   -   A) reacting an N-protected amino acid ester with hydrazine to        form an N-protected acylhydrazide;    -   B) reacting the N-protected acylhydrazide with a        chloro-oxo-acetic acid ester in the presence of a first base to        afford an N-protected diacylhydrazide;    -   C) cyclizing the N-protected diacylhydrazide in the presence of        a dehydrating reagent and a second base to afford the        N-protected 1,3,4-oxadiazole;    -   D) deprotecting the N-protected 1,3,4-oxadiazole to form the        product.

1,2,4-Oxadiazoles and 1,3,4-oxadiazoles are widely used as ester andamide bioisosteres. They are also useful as antiviral agents,neuroprotectants, and anti-inflammatory agents, or precursors thereof.

DETAILED DESCRIPTION OF THE INVENTION

According to Scheme 1, the present invention relates to a method for theformation of 1,2,4-oxadiazoles (v) by treating an amidoxime (ii) and ananhydride (iv) with a base. The resulting product (v), where R₂ is aprotecting group, and preferably a BOC protecting group, is thendeprotected using hydrogen chloride in ethyl acetate to afford the finalproduct (v) where R₂ is hydrogen.

In Scheme 1:

-   -   m and n independently are 0 to 6;    -   R₁ is C₁-C₆ alkyl or arylalkyl, where each aryl is optionally        substituted with one, two or three groups independenly selected        from C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen, nitro, trifluoromethyl,        and trifluoromethoxy; and    -   R₂ is a suitable amino acid protecting group; and    -   each R₃ is independently selected from hydrogen, a suitably        protected amino acid residue, or heterocycloalkyl optionally        substituted with C₁-C₆ alkyl, or        -   C₁-C₆ alkyl optionally substituted with one or two groups            independently selected from thioalkyl, hydroxy, C₁-C₆            alkoxy, carboxamido, mono or di(C₁-C₆ alkyl)carboxamido,            heterocycloalkyl, amidinyl, mono or di(C₁-C₆)amino,            protected amino, protected carboxyl, and            -   phenyl, optionally substituted with one or two groups                independently selected from C₁-C₆ alkoxy and C₁-C₆                alkyl.

In a preferred embodiment, R₁ is C₁-C₆ alkyl or benzyl, R₂ istertiary-butoxycarbonyl, and R₃ is an amino acid residue or C₁-C₆ alkyl.

The amidoxime (ii) can be purchased or prepared, such as, for example,by treating a cyanocarboxylate (i) with hydroxylamine or a salt thereof,and a base in a solvent, as depicted in Scheme 2.

wherein n and R₁ are as defined above for Scheme 1.

In preferred embodiment of Scheme 2, the hydroxylamine salt is selectedfrom hydroxylamine hydrochloride, hydroxylamine nitrate, hydroxylaminesulfate, aqueous hydroxylamine, and hydroxylamine phosphate.

Another embodiment of the present invention relates to a method for theformation of 1,3,4-oxadiazoles. Accordingly, a hydrazide (viii) isacylated with an acylating agent and a base in a non-anhydrous solventto afford the diacylhydrazide (ix). The diacylhydrazide is then cyclizedusing triphenylphosphine, carbon tetrachloride and a base in a solventto form the protected 1,3,4-oxadiazoles (x), where R₂ istert-butoxycarbonyl (BOC.) The protected 1,3,4-oxadiazole (x) is thendeprotected using hydrogen chloride in ethyl acetate or dioxane.

According to Scheme 3:

-   -   z is 1 to 6;    -   each R₄ is independently selected from hydrogen, a suitably        protected amino acid residue, or heterocycloalkyl optionally        substituted with C₁-C₆ alkyl, or        -   C₁-C₆ alkyl optionally substituted with one or two groups            independently selected from thioalkyl, hydroxy, C₁-C₆            alkoxy, carboxamido, mono or di(C₁-C₆ alkyl)carboxamido,            heterocycloalkyl, amidinyl, mono or di(C₁-C₆)amino,            protected amino, protected carboxyl, and            -   phenyl, optionally substituted with one or two groups                independently selected from C₁-C₆ alkoxy and C₁-C₆                alkyl;    -   R₅ is C₁-C₆ alkyl or arylalkyl, where each aryl is optionally        substituted with one, two or three groups independenly selected        from C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen, nitro, trifluoromethyl,        and trifluoromethoxy; and    -   R₆ is a suitable amino acid protecting group.

In a preferred embodiment of Scheme 3, R₅ is lower alkyl, and morepreferably, a C₁-C₄ alkyl group. Most preferably, the C₁-C₄ alkyl groupis ethyl.

Examples of acceptable first bases used in the present method are thosewith alkali metals or alkaline earth metals such as sodium, potassium,calcium and magnesium, and those with organic bases including, but notlimited to, amines. Preferred bases are alkali metal bases or alkalineearth metal bases. Even more preferred bases are alkaline metalcarbonates, such as, for example, sodium carbonate, potassium carbonate,lithium carbonate, calcium carbonate, and cesium carbonate.

In still yet another preferred embodiment, the cyanocarboxylate istreated with a hydroxylamine salt and a first base at a temperature offrom between 0° C. and 75° C. Most preferably, the temperature is about15° C. to about 30° C.

In another preferred embodiment, the cyanocarboxylate is treated with ahydroxylamine salt and a first base in at least one suitable solvent.More preferably, the solvent is protic, including for example, alcohols,acids, water and mixtures thereof. Even more preferably, the proticsolvent is a combination of ethanol and water.

Suitably protected-amino acid and/or amino acid residues of the presentinvention include, but are not limited to, N-tertiary-butoxycarbonylglycine, N-tertiary-butoxycarbonyl alanine, N-tertiary-butoxycarbonylvaline, N-tertiary-butoxycarbonyl leucine, N-tertiary-butoxycarbonylmethionine, N-tertiary-butoxycarbonyl isoleucine,N-tertiary-butoxycarbonyl serine, N-tertiary-butoxycarbonyl threonine,N-tertiary-butoxycarbonyl cysteine, N-tertiary-butoxycarbonyl proline,N-tertiary-butoxycarbonyl asparagine, N-tertiary-butoxycarbonylglutamine, N-tertiary-butoxycarbonyl phenylalanine,N-tertiary-butoxycarbonyl tyrosine, N-tertiary-butoxycarbonyltryptophan, N-tertiary-butoxycarbonyl lysine, N-tertiary-butoxycarbonylarginine, N-tertiary-butoxycarbonyl histidine, N-tertiary-butoxycarbonylaspartine, and N-tertiary-butoxycarbonyl glutamine. It is understoodthat any such protected-amino acid may have further reactive sidechains.In such instances, the reactive sidechains are also suitably protected.

In still another preferred embodiment, the amidoxime intermediate istreated with the acid anhydride intermediate in the presence of a secondbase at a temperature of from between 0° C. and 125° C. Even morepreferably, the temperature is between 20° C. and 125° C.

The second base is preferably an organic base. More preferably, theorganic base is anhydrous. Even more preferably, the anhydrous, organicbase is aromatic, including, for example, lutidine, pyridine, collidineand 2,6-di-tertiary-butyl pyridine. Most preferably, the second base isanhydrous pyridine.

In still another preferred embodiment, deprotecting the 1,2,4-oxadiazolecarboxylate product by treatment with an appropriate deprotecting agentis performed at a temperature of from between −10° C. and 85° C. Morepreferably, the temperature is from about 0° C. to about 85° C.

Examples of deprotecting reagents include, but are not limited to,trifluoroacetic acid, acetic acid, hydrogen chloride, hydrogen bromide,hydrogen iodide, triflic acid, and methanesulfonic acid. Preferably, theprotic reagent is hydrogen chloride gas. Even more preferably, theprotic reagent is hydrogen chloride gas dissolved in either 1,4 dioxane(“dioxane”), ethyl acetate or diethylether.

In another preferred embodiment, the deprotection is carried out in atleast one suitable solvent. More preferably, the solvent is aprotic,including for example, ethers, halogenated hydrocarbons, esters andsulfoxides. Even more preferably, the aprotic solvent is ethyl acetate.

In a preferred embodiment, the nitrogen protecting group is acid labile.More preferably, the nitrogen protecting group istertiary-butoxycarbonyl (BOC.)

In another preferred embodiment, the N-protected acylhydrazide istreated with a chloro-oxo-acetic acid ester in the presence of at leastone solvent. More preferably, the solvent is aprotic, including forexample, ethers, esters and sulfoxides. Even more preferably, thesolvent is tetrahydrofuran.

In yet another preferred embodiment, the diacylhydrazide is formed attemperatures between −10° C. and 40° C.

The cyclization of the N-protected diacylhydrazide can also be performedin the presence of at least one solvent. Preferably, the solvent is ahalogenated hydrocarbon, including, for example, trichloroethane,methylene chloride, chloroform and carbon tetrachloride. Even morepreferably, the solvent is dichloromethane.

Examples of a suitable second base include, but are not limited to,triethylamine, diisopropylethylamine, pyridine, and lutidine. Morepreferably, the second base is triethylamine.

The cyclization of the N-protected diacylhydrazide can also be performedin the presence of a phosphorous reagent combined with a per-halogenatedmethane. Preferably, the phosphorous reagent is triphenylphosphine andthe perhalogenated methane is carbon tetrachloride.

In another preferred embodiment, the cyclization of the N-protecteddiacylhydrazide is performed at temperature of about 15° C. to about 90°C.

The N-protected 1,3,4-oxadiazole can be deprotected using reagents suchas, for example, trifluoroacetic acid, acetic acid, hydrogen chloride,hydrogen bromide, hydrogen iodide, triflic acid, and methanesulfonicacid. More preferably, the protic reagent is hydrogen chloride gas. Evenmore preferably, the protic reagent is hydrogen chloride gas dissolvedin dioxane, ethyl acetate or diethylether.

In a preferred embodiment, the N-protected 1,3,4-oxadiazole isdeprotected in the presence of at least one solvent. More preferably,the solvent is aprotic, including for example, ethers, halogenatedhydrocarbons, esters and sulfoxides. Even more preferably, the solventis ethyl acetate.

In another preferred embodiment, the deprotection of the nitrogen isperformed at temperature of about −10° C. to about 15° C.

By “alkoxy” in the present invention is meant straight or branched chainalkoxy groups having 1-6 carbon atoms, such as, for example, methoxy,ethoxy, propoxy, isopropoxy, n-butoxy, sec-butoxy, tert-butoxy, pentoxy,2-pentyl, isopentoxy, neopentoxy, hexoxy, 2-hexoxy, 3-hexoxy, and3-methylpentoxy. These groups may be substituted with up to four groupsmentioned below for aryl.

By “alkyl” in the present invention is meant straight or branched chainalkyl groups having 1-6 carbon atoms, such as, for example, methyl,ethyl, propyl, isopropyl, n-butyl, sec-butyl, iso-butyl, tert-butyl,pentyl, 2-pentyl, isopentyl, neopentyl, hexyl, 2-hexyl, and 3-hexyl.

By “aryl” is meant an aromatic carbocyclic group having a single ring(e.g., phenyl), multiple rings (e.g., biphenyl), or multiple condensedrings in which at least one is aromatic (e.g.,1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl), which isoptionally mono-, di-, or trisubstituted with halogen, —OH, —SH, alkyl,alkoxy, alkylthio, trifluoromethyl, trifluoromethoxy, acyloxy, aryl,heteroaryl, amino, mono-or dialkylamino, and nitro. A preferred arylgroup is phenyl.

A “cycloalkyl” group is a nonaromatic cyclic ring or fused rings havingfrom 3 to 7 members. Examples include cyclopropyl, cyclobutyl, andcycloheptyl. These rings may be substituted with one or more of thesubstituent groups mentioned below for aryl, for example, alkyl, halo,amino, hydroxy, and alkoxy. Typical substituted carbocyclic groupsinclude 2-chlorocyclopropyl; 2,3-diethoxycyclopentyl; and2,2,4,4-tetrafluorocyclohexyl. The carbocyclic group may contain one ortwo heteroatoms selected from oxygen, sulfur and nitrogen, and such ringsystems may be referred to as “heterocyclic.” Examples include pyranyl,tetrahydrofuranyl, and dioxanyl. These heterocyclic groups may besubstituted with up to four of the substituent groups mentioned for arylto gives groups such as 3-chloro-2-dioxanyl, and3,5-dihydroxymorpholino. In addition, the carbocyclic or heterocyclicgroup may also contain one or more internal double bonds, as long ashaving such double bonds does not make the carbocycle or heterocyclearomatic.

The terms “halogen” or “halo” indicate fluorine, chlorine, bromine, andiodine.

The term “heterocycloalkyl,” refers to a non-aromatic ring systemcontaining at least one heteroatom selected from nitrogen, oxygen, andsulfur. The heterocycloalkyl ring may be optionally fused to orotherwise attached to other heterocycloalkyl rings and/or non-aromatichydrocarbon rings. Preferred heterocycloalkyl groups have from 3 to 7members. Examples of heterocycloalkyl groups include, for example,piperazine, morpholine, piperidine, tetrahydrofuran, pyrrolidine, andpyrazole. Preferred heterocycloalkyl groups include piperidinyl,piperazinyl, morpholinyl, and pyrolidinyl.

The term “protecting group”, “suitably protected” or “nitrogenprotecting group,” as used herein, refers to groups known in the artthat are readily introduced on to and removed from a nitrogen atom.Examples of nitrogen protecting groups include tertiary-butoxy carbonyl(BOC), carbobenzyloxy (Cbz), benzoyl, and benzyl. Other examples ofacceptable nitrogen protecting groups are found in “Protective Groups inOrganic Synthesis”, 3rd Ed., Greene, T. W.

The compounds of this invention may contain one or more asymmetriccarbon atoms, so that the compounds can exist in differentstereoisomeric forms. These compounds can be, for example, racemates,chiral non-racemic or diastereomers. In these situations, the singleenantiomers, i.e., optically active forms can be obtained by asymmetricsynthesis, using enantiomerically enriched starting materials or byresolution of the racemates. Resolution of the racemates can beaccomplished, for example, by conventional methods such ascrystallization in the presence of a resolving agent; chromatography,using, for example a chiral HPLC column; or derivatizing the racemicmixture with a resolving reagent to generate diastereomers, separatingthe diastereomers via chromatography, and removing the resolving agentto generate the original compound in enantiomerically enriched form. Anyof the above procedures can be repeated to increase the enantiomericpurity of a compound.

Scheme 4 depicts an example for the formation of the acid anhydride(iv), which is used in the present invention for the preparation of1,2,4-oxadiazoles. Specifically, (iv) can be prepared by treating asuitably protected amino acid (iii) with a dehydrating agent in asolvent.

In Scheme 4, R₂, R₃ and m are as defined above.

Scheme 5 exemplifies the preparation of the protected acyl hydrazide(vii), which is used in the preparation of 1,3,4-oxadiazoles. Usingmeans well known in the art, amino-ester (vi) is treated with aprotecting group precursor to form the protected amino-ester (vii),which is then treated with hydrazine to form the acyl hydrazide (viii.)

In Scheme 5, z, R₄ and R₆ are as defined above.

The disclosures in this application of all articles and references,including patents, are incorporated herein by reference.

The invention is illustrated further by the following examples, whichare not to be construed as limiting the invention in scope or spirit tothe specific procedures described in them.

The starting materials and various intermediates may be obtained fromcommercial sources, prepared from commercially available organiccompounds, or prepared using well-known synthetic methods.

Representative examples of methods for preparing intermediates of theinvention are set forth below.

EXAMPLES Example 1 Synthesis of Anhydrides 1.3-[(tert-Butoxy)carbonylamino]propanoyl3-[(tert-butoxy)carbonylamino]propanoate (1a)

A solution of N,N-dicyclohexylcarbodiimide (DCC) (109 g, 0.529 mol) indry methylene chloride (200 ml) is added dropwise to a 0° C. solution ofBOC protected β-alanine (200 g, 1.058 mol) in dry methylene chloride(800 ml.) After the addition is complete, the reaction mixture isstirred for 2 hours. The resulting N,N-dicyclohexylurea (DCU) is removedvia filtration, and the filtrate is concentrated in vacuo to yield thecrude anhydride 1a, which is used directly in the next step withoutfurther purification.

Example 2 Synthesis of Amidoximes 1. Ethyl2-amino-2-(hydroxyimino)acetate (2a)

Water (600 ml) is added dropwise over a period of two hours to avigorously stirred, room temperature mixture of ethyl cyanoformate (99g, 1 mol), hydroxylamine hydrochloride (105 g, 1.5 mol ) and sodiumcarbonate (82 g, 0.77 mol ) in ethanol (1 L.) The resulting solution isstirred until the starting material has been consumed. When the reactionis complete, most of the solvent is removed in vacuo and the resultingresidue is extracted with methylene chloride (3×200 ml.) The combinedorganic extracts are washed with brine (250 ml), dried (Na₂SO₄),filtered and concentrated to afford 120.1 g (91%) of compound 2a as awhite solid. Further purification can be achieved by crystallizationfrom chloroform and hexanes. ¹H NMR (CDCl₃): δ 9.15(br s, OH), 5.12(brs, 2H), 4.32(q, 2H), 1.42(t, J=7.1 Hz, 3H). ¹³C NMR (CDCl₃): δ 161.4,144.4, 62.8, 14.3.

2. Ethyl 3-amino-3-(hydroxyimino)propanoate (2b)

Water (500 ml) is added dropwise over a period of 2 hours to avigorously stirred, room temperature mixture of ethyl cyanoacetate (100g, 0.884 mol), hydroxylamine hydrochloride (3 g, 1.328 mol) and sodiumcarbonate (70 g, 0.66 mol ) in ethanol (1 L.) After stirring for anadditional 12 hours at room temperature, the reaction mixture is heatedto 50° C. for 1 hour. The resulting light red colored solution isallowed to cool to room temperature and then stirred for an additional 2hours. The solids formed are removed via filtration and the filtrate isconcentrated in vacuo. The resulting residue is extracted with methylenechloride (3×200 ml). The combined organic extracts are washed with brine(300 ml), dried (Na₂SO₄), filtered and concentrated to afford 85.2 g(66%) of compound 2b as a slightly colored solid. Compound 2b can befurther purified by crystallization from chloroform and hexanes. ¹H NMR(CDCl3): δ 8.45 (br s, 1H), 5.25(br s, 2H), 4.25(q, 2H), 3.15(s, 2H),1.35(t, J=7 Hz, 3H).

Example 3 Synthesis of 1,2,4-Oxadiazoles 1. Ethyl5-{2-[(tert-butoxy)carbonylamino]ethyl}-1,2,4-oxadiazole-3-carboxylate(3a)

Amidoxime 2a (46.6 g, 0.353 mol) in dry pyridine (100 ml) is addeddropwise to a room temperature solution of crude anhydride 1a in drypyridine (300 ml.) The reaction mixture is then refluxed for 6 hours.During this time, the reaction is monitored by TLC on silica gel usingether/hexanes (2:1) as the eluent. Upon consumption of the startingmaterial, the reaction mixture is cooled to room temperature and thenwater (200 ml) is added. The solvent is evaporated in vacuo, and theresulting residue is dissolved in methylene chloride (300 ml) and washedsequentially with water (200 ml), saturated sodium bicarbonate (3×100ml) and brine (100 ml). The organic phase is dried over sodium sulfate,filtered, and concentrated in vacuo to afford the crude product. Thecrude product is then filtered through a pad of silica gel usinghexanes/ether (2:1) as the eluent to afford 71 g (78%) of the pureprotected oxadiazole 3a as a white crystalline solid. Furtherpurification can be effected, if desired, by crystallization fromchloroform and hexanes. ¹H NMR (CDCl₃): δ 5.21(bs s, 2H), 4.52 (q, 2H),1.62(d, J=6.94 Hz, 3H), 1.44(m, 12H). ¹³C NMR (D₂O): δ 182.5, 162.0,157.8, 155.1, 80.7, 63.3, 44.5, 28.5, 19.9, 14.3.

2. Ethyl 5-(2-aminoethyl)-1,2,4-oxadiazole-3-carboxylate (4a)

Dry hydrogen chloride gas is bubbled through a 0° C. solution ofprotected oxadiazole 3a (50 g) in ethyl acetate (750 ml) until a whitesolid begins to precipitate (approximately 15 minutes.) Then thesolution is refluxed for 10 hours, cooled to room temperature, andfiltered. The resulting solid is washed with ether and dried to afford41 g (96 %) of compound 4a as a white solid. ¹H NMR (D₂O): δ 4.40(q,2H), 3.55(m, 4H), 1.32(t, 3H). ¹³C NMR (D₂0): δ 179.1, 161.8, 158.6,64.6, 36.2, 24.8, 13.7.

Example 4 Boc Protection 1. Ethyl 2-[(tert-butoxy)carbonylamino]acetate(5)

Di-tert-butyl dicarbonate (365 g, 1.67 mol) in tetrahydrofuran (1 L) isadded dropwise to a 0° C. solution of glycine ethyl ester hydrochloride(232.5 g, 1.66 mol) and triethylamine (497.5 ml, 3.57 mol) intetrahydrofuran (2.5 L.) The reaction mixture is vigorously stirred for36 hours and then filtered. The filtrate is the concentrated in vacuo toafford 329.4 g (98%) of compound 5 as a white solid.

Example 5 Synthesis of Hydrazides 1. Hydrazinocarbonylmethyl-carbamicacid tert-butyl ester (6a)

Hydrazine hydrate (222 ml, 6.92 mol) is added dropwise to a roomtemperature solution of compound 5 (285.7 g, 1.63 mol) in ethanol (500ml). The initial reaction is mildly exothermic. The reaction is stirredfor 36 hours and then the volatiles are removed in vacuo. Water (400 ml)is added to the crude product, briefly stirred, and then removed invacuo in order to azeotropically remove excess hydrazine. Pentane isadded to the resulting white solid, which is broken up by heatinggently. After cooling to approximately room temperature, the mixture isfiltered and the solid is washed with pentane to afford 236.7 g (90%) ofcompound 6 as a white solid.

Example 6 Synthesis of Diacylhydrazides 1. Ethyl(N-{2-[(tert-butoxy)carbonylamino]acetylamino}carbamoyl)formate (7)

Ethyl oxalyl chloride (81 ml, 0.72 mol) is added dropwise over a periodof 90 minutes to a vigorously stirred, 0° C. slurry of compound 6 (136g, 0.72 mol) and sodium hydrogen carbonate (69 g, 0.82 mol) intetrahydrofuran (2.5 L.) The reaction mixture is allowed to warm to roomtemperature and then stirred for an additional 12 hours. The reactionmixture is then filtered through celite and the filtrate is concentratedin vacuo. Toluene (500 ml) is added to the resulting residue, and thenvolume of solvent is reduced in vacuo to approximately half its originalvolume. Addition of ethyl ether (1 L) causes some of the product toprecipitate as a white solid. The flask is then placed in the freezerovernight. The resulting white solid is collected by vacuum filtrationand washed with ethyl ether (500 mL) to afford 114.6 g (55%) of compound7 as a white solid.

Example 7 Synthesis of 1,3,4-Oxadiazoles 1. Ethyl5-{[(tert-butoxy)carbonylamino]methyl}-1,3,4-oxadiazole-2-carboxylate(8)

A solution of compound 7 (59 g, 200 mmol) and triethylamine (42 ml, 0.57mol) in carbon tetrachloride (60 ml) is added to a stirred, roomtemperature of triphenylphosphine (63 g, 0.24 mol) in methylene chloride(900 ml). The reaction mixture is stirred for 30 minutes and thenrefluxed for 12 hours. After cooling to room temperature, the volatilesare removed in vacuo and the resulting residue is filtered throughsilica using methylene chloride:ethyl acetate (9:1) as the eluent. Afterremoving the volatiles in vacuo, the crude product is purified by flashcolumn chromatography over silica gel using methylene chloride:ethylacetate (19:1) as the eluent. 24 g (44%) Of compound 8 is obtained asorange oil, which solidifies to a yellow solid on standing.

2. Ethyl 5-(aminomethyl)-1,3,4-oxadiazole-2-carboxylate (9)

Dry hydrogen chloride gas is bubbled through ethyl acetate (500 ml) at0° C. for 10 minutes. Compound 8 (40.6 g, 0.15 mol) is then addedportionwise over 5 minutes. Vigorous gas evolution is noticed and thesolution quickly turns cloudy. After stirring for 2 hours, the whitesolid is collected by vacuum filtration and washed exhaustively withdiethyl ether (1 L) to afford 27.2 g (86 %) of compound 9. ¹H NMR (D₂O):δ 4.56 (s, 2H), 4.40 (q, 2H), 1.28(t, 3H). ¹³C NMR (D₂0): δ 162.9,158.5, 160.0, 65.2, 34.4, 13.5.

In an alternative procedure, compound 8 (33.3 g, 0.12 mol) is addedportionwise to a 0° C. commercially available solution of hydrogenchloride in 1,4-dioxan (Aldrich, 4 M, 750 ml.) Compound 8 slowlydissolves to afford a clear yellow solution. After stirring forapproximately 30 minutes, the reaction mixture turns cloudy. Thestarting material is consumed in about two hours, as determined by TLCon silica using methylene chloride:ethyl acetate (9:1) as the eluent.The reaction mixture is filtered and a colorless solid product isobtained. The solid is washed exhaustively with ether (1 L) to afford19.2 g (76 %) of compound 9.

Alternatively, the deprotection can be performed on the crude productfrom the cyclization step.

Crude, unpurified compound 8 (5 g) is added portionwise to a 0° C.commercially available solution of hydrogen chloride in 1,4-dioxan(Aldrich, 4 M, 35 ml.) Compound 8a slowly dissolves to afford a clearyellow solution. After stirring for approximately 12 hours, the reactionmixture turns cloudy. TLC on silica using methylene chloride:ethylacetate (9:1) as the eluent shows the starting material has beenconsumed. The reaction mixture is filtered and a white solid isobtained. The solid is washed exhaustively with ether (50 mL) to afford266 mg of compound 9.

Example 8

The compounds depicted in Tables 1 and 2 are essentially preparedaccording to the procedures described above in Examples 1-7. TABLE 11,2,4-oxadiazoles. Amount prepared (grams); Compound Total Yield (%) NMRData Name

107; (87) ¹H NMR (D₂O): δ 4.40(q, 2H), 3.55(m, 4H), 1.32(t, 3H). ¹³C NMR(D₂O): δ 179.1, 161.8, 158.6, 64.6, 36.2, 24.8, 13.7. 5-(2-Amino-ethyl)-[1,2,4]oxadiazole-3-carboxylic acid ethyl ester dihydro chloride

107; (78) ¹H NMR (D₂O): δ 4.65 (s, 2H), 4.25(q, 2H), 3.95(s, 2H),1.25(t, J=7.1 Hz, 3H). ¹³C NMR (D₂O): δ 173.7, 170.2, 165.3, 63.4, 35.5,32.0, 13.7. (5-Aminomethyl- [1,2,4]oxadiazol-3-yl)-acetic acid ethylester dihydro chloride

 85; (82) ¹H NMR (D₂O): δ 4.65(s, 2H), 4.42(q, 2H), 1.32(t, J=7.3 Hz,3H). ¹³C NMR (D₂O): δ 175.0, 162.0, 158.4, 64.7, 35.5, 13.6.5-Aminomethyl- [1,2,4]oxadiazole-3-carboxylic acid ethyl ester dihydrochloride

 50; (76) ¹H NMR (D₂O): δ 5.05(q, 2H), 4.40(q, 2H), 1.75(d, J=7.06 Hz,2H), 1.25(t, J=7.13 Hz, 3H). ¹³C NMR (D₂O): δ 178.0, 162.0, 158.3, 64.7,44.3, 16.5, 13.5. 5-(1-Amino-ethyl)- [1,2,4]oxadiazole-3-carboxylic acidethyl ester dihydro chloride

TABLE 2 1,3,4-oxadiazoles prepared using new methodology. Amountprepared (grams); Compound Total Yield (%) NMR Data Name

79; (22) ¹H NMR (D₂O): δ 4.56(s, 2H), 4.40(q, 2H), 1.28(t, 3H). ¹³C NMR(D₂O): δ 162.9, 158.5, 160.0, 65.2, 34.4, 13.5. 5-Aminomethyl-[1,3,4]oxadiazole-2- carboxylic acid ethyl ester hydro chloride

The invention and the manner and process of making and using it, are nowdescribed in such full, clear, concise and exact terms as to enable anyperson skilled in the art to which it pertains, to make and use thesame. It is to be understood that the foregoing describes preferredembodiments of the present invention and that modifications may be madetherein without departing from the spirit or scope of the presentinvention as set forth in the claims. To particularly point out anddistinctly claim the subject matter regarded as invention, the followingclaims conclude this specification.

1. A method for preparing a 1,3,4-oxadiazole carboxylate comprising: A)reacting the N-protected acylhydrazide with a chloro-oxo-acetic acidester in the presence of a first base to afford an N-protecteddiacylhydrazide; B) cyclizing the N-protected diacylhydrazide in thepresence of a phosphorous reagent, a per-halogenated methane and asecond base to afford the N-protected 1,3,4-oxadiazole; C) deprotectingthe N-protected 1,3,4-oxadiazole to form the product.
 2. A methodaccording to claim 1, wherein the 1,3,4-oxadiazole is

wherein z is 1 to 6; each R₄ is independently selected from hydrogen, asuitably protected amino acid residue, or heterocycloalkyl optionallysubstituted with C₁-C₆ alkyl, or C₁-C₆ alkyl optionally substituted withone or two groups independently selected from thioalkyl, hydroxy, C₁-C₆alkoxy, carboxamido, mono or di(C₁-C₆ alkyl)carboxamido,heterocycloalkyl, amidinyl, mono or di(C₁-C₆)amino, protected amino,protected carboxyl, and phenyl, optionally substituted with one or twogroups independently selected from C₁-C₆ alkoxy and C₁-C₆ alkyl; R₅ isC₁-C₆ alkyl or arylalkyl, where each aryl is optionally substituted withone, two or three groups independently selected from C₁-C₆ alkyl, C₁-C₆alkoxy, halogen, nitro, trifluoromethyl, and trifluoromethoxy; and R₆ isa suitable amino acid protecting group.
 3. A method according to claim 1wherein the tertiary-butoxycarbonyl protected-amino acid is selectedfrom N-tertiary-butoxycarbonyl glycine, N-tertiary-butoxycarbonylalanine, N-tertiary-butoxycarbonyl valine, N-tertiary-butoxycarbonylleucine, N-tertiary-butoxycarbonyl methionine, N-tertiary-butoxycarbonylisoleucine, N-tertiary-butoxycarbonyl serine, N-tertiary-butoxycarbonylthreonine, N-tertiary-butoxycarbonyl cysteine, N-tertiary-butoxycarbonylproline, N-tertiary-butoxycarbonyl asparagine, N-tertiary-butoxycarbonylglutamine, N-tertiary-butoxycarbonyl phenylalanine,N-tertiary-butoxycarbonyl tyrosine, N-tertiary-butoxycarbonyltryptophan, N-tertiary-butoxycarbonyl lysine, N-tertiary-butoxycarbonylarginine, N-tertiary-butoxycarbonyl histidine, N-tertiary-butoxycarbonylaspartine, and N-tertiary-butoxycarbonyl glutamine, wherein if theprotected-amino acid has further reactive sidechains, such sidechainsare protected with a suitable protecting group.
 4. A method according toclaim 1 wherein the first base is selected from sodium bicarbonate,sodium carbonate, potassium carbonate, calcium carbonate, bariumcarbonate, cesium carbonate, and strontium carbonate.
 5. A methodaccording to claim 4 wherein the first base is sodium bicarbonate.
 6. Amethod according to claim 1 wherein R5 is a C₁-C₄ alkyl group.
 7. Amethod according to claim 6 wherein the C₁-C₆ alkyl group is ethyl.
 8. Amethod according to claim 1 wherein the N-protected acylhydrazide istreated with a chloro-oxo-acetic acid ester in the presence of at leastone solvent.
 9. A method according to claim 8 wherein the solvent isaprotic.
 10. A method according to claim 9 wherein the solvent istetrahydrofuran.
 11. A method according to claim 1 wherein thediacylhydrazide is formed at temperatures between −10° C. and 40° C. 12.A method according to claim 1 wherein the cyclization of the N-protecteddiacylhydrazide is performed in the presence of at least one solvent.13. A method according to claim 12 wherein the solvent is a halogenatedhydrocarbon.
 14. A method according to claim 13 wherein the solvent isdichloromethane.
 15. A method according to claim 1 wherein the secondbase is selected from triethylamine, diisopropylethylamine, pyridine,and lutidine.
 16. A method according to claim 15 wherein the second baseis triethylamine.
 17. A method according to claim 1 wherein thedehydrating agent is a phosphorous reagent combined with aperhalogenated methane.
 18. A method according to claim 17 wherein thephosphorous reagent is triphenylphosphine and the perhalogenated methaneis carbon tetrachloride.
 19. A method according to claim 1 wherein thedehydration is performed at temperature of about 15° C. to about 90° C.20. A method according to claim 1 wherein the N-protected1,3,4-oxadiazole is deprotected using a protic reagent.
 21. A methodaccording to claim 20 wherein the protic reagent is hydrogen chloridegas.
 22. A method according to claim 20 wherein the protic reagent ishydrogen chloride gas dissolved in 1,4-dioxane.
 23. A method accordingto claim 1 wherein the N-protected 1,3,4-oxadiazole is deprotected inthe presence of at least one solvent.
 24. A method according to claim 23wherein the solvent is aprotic.
 25. A method according to claim 24wherein the solvent is ethyl acetate.
 26. A method according to claim 25wherein the deprotection is performed at temperature of about −10° C. toabout 15° C.
 27. A method according to claim 20 wherein the deprotectionis performed without the addition of any additional solvent.
 28. Amethod according to claim 27 wherein the deprotection is performed attemperature of about −10° C. to about 30° C.
 29. A method for preparingcompounds of the formula:

wherein z is 1 to 6; each R₄ is independently selected from hydrogen, asuitably protected amino acid residue, or heterocycloalkyl optionallysubstituted with C₁-C₆ alkyl, or C₁-C₆ alkyl optionally substituted withone or two groups independently selected from thioalkyl, hydroxy, C₁-C₆alkoxy, carboxamido, mono or di(C₁-C₆ alkyl)carboxamido,heterocycloalkyl, amidinyl, mono or di(C₁-C₆)amino, protected amino,protected carboxyl, and phenyl, optionally substituted with one or twogroups independently selected from C₁-C₆ alkoxy and C₁-C₆ alkyl; R₅ isC₁-C₆ alkyl or arylalkyl, where each aryl is optionally substituted withone, two or three groups independenly selected from C₁-C₆ alkyl, C₁-C₆alkoxy, halogen, nitro, trifluoromethyl, and trifluoromethoxy; and R₆ isa suitable amino acid protecting group; the method comprising A)reacting an intermediate of the structure

wherein z, R₄ and R₆ are defined above, with

in the presence of a first base and isolating a diacylhydrazideintermediate of the formula:

wherein z, R₄, R₅, and R₆ are defined above; B) cyclizing thediacylhydrazide intermediate in the presence of a phosphorous reagent, aper-halogenated methane and a second base to afford an intermediate ofthe formula:

wherein z, R₄, R₅, and R₆ are defined above; C) deprotecting thecyclized intermediate to form a compound of the formula:


30. A method according to claim 29 wherein the phosphorous reagent istriphenylphosphine and the perhalogenated methane is carbontetrachloride.
 31. A method according to claim 29 wherein R₆ istertiary-butoxy carbonyl.
 32. A method according to claim 29 wherein R₅is C₁-C₆ alkyl.